Cardiovascular disease is a leading cause of death, and as a result, the medical community has devised various methods and devices for the treatment of coronary heart disease including those associated with the complications resulting from atherosclerosis or other forms of coronary arterial closing or narrowing. One such treatment utilized in cases involving atherosclerosis and/or other forms of coronary narrowing is referred to as percutaneous transluminal coronary angioplasty, sometimes simply referred to as angioplasty or PTCA. The objective of this technique is to radially enlarge the lumen of the impacted artery. This is accomplished by first positioning an expandable balloon in a target lesion (i.e., the narrowed lumen of the coronary artery). Inflation of the balloon causes (1) soft or fatty plaque deposits to be flattened by the balloon and (2) hardened deposits to crack and split thereby enlarging the lumen. In addition, the artery wall itself is stretched by the inflated balloon.
In a typical percutaneous transluminal coronary angioplasty (PTCA) procedure, a hollow guiding catheter is introduced into the cardiovascular system of a patient via a relatively large vessel such as the femoral artery in the groin area or the brachial artery in the arm. After access to the patient's cardiovascular system has been achieved, a short hollow sheath is inserted to maintain the passageway during the procedure. After the guiding catheter reaches the ostium of the coronary artery to be treated by angioplasty, a flexible guide wire and a dilatation catheter having a balloon on the distal end thereof are introduced into the guide catheter with the guide wire sliding through the dilatation catheter. The guide wire is advanced through a target lesion in the vasculature. A balloon or dilatation catheter (made of, for example, polyethylene, polyethylene terathalate, PEBAX (polyamide block copolymers and polyester block copolymers), polyvinyl chloride, polyolefin, nylon, or other suitable substance) is then slidably advanced over the previously advanced guide wire by sliding it along the guide wire until the dilatation balloon is properly positioned across the target lesion. Radiopaque markers in the balloon portion of the dilatation catheter assist in the positioning of the balloon across the lesion. After proper positioning, the balloon is inflated, generally with a contrast material to permit fluoroscopic viewing during the treatment, so as to enlarge the lumen of the artery. Treatment may require that the balloon be alternately inflated and deflated until satisfactory enlargement has been achieved. The balloon is then deflated to a small profile so that the dilatation catheter may be withdrawn from the patient's vasculature and blood flow resumed through the dilated artery. Unfortunately, after angioplasty procedures of this type, there may occur a restenosis of the artery; i.e. a renarrowing of the treated coronary artery that significantly diminishes any positive results of the angioplasty procedure. In the past, restenosis frequently necessitated repeat PTCA or even more drastic open-heart surgery.
To prevent restenosis and strengthen the target area, various devices have been proposed for mechanically keeping the affected vessel open after completion of the angioplasty procedure. Such mechanical endoprosthetic devices, generally referred to as stents, are typically inserted into the vessel, positioned across the target lesion, and then expanded to keep the lumen clear. A stent is mounted in a compressed state around a deflated balloon, and the balloon/stent assembly maneuvered through a patient's vasculature to the site of a target lesion. After positioning, the balloon is inflated causing the stent to be expanded to a larger diameter for placement or implantation in the vasculature. The stent effectively overcomes the natural tendency of the vessel walls of some patients to close back down, thereby permitting an increased flow of blood through the vessel that would not be possible if the stent were not in place.
Stents are generally tubular shaped devices which function to hold open a segment of blood vessel or other anatomical lumen. To be effective, the stent should be relatively flexible along its length so as to facilitate delivery through torturous body lumens, and yet stiff and stable enough when radially expanded to maintain the blood vessel or artery open. Such stents may include a plurality of axial bends or crowns adjoined together by a plurality of struts so as to form a plurality of U-shaped members coupled together to form a serpentine pattern.
Stents may be formed using any of a number of different methods. One such method involves forming segments from rings, welding or otherwise forming the stent to a desired configuration, and compressing the stent to an unexpanded diameter. Another such method involves machining tubular or solid stock material into bands and then deforming the bands to a desired configuration. While such structures can be made many ways, one low cost method is to cut a thin-walled tubular member of a biocompatible material (e.g. stainless steel, titanium, tantalum, super-elastic nickel-titanium alloys, high-strength thermoplastic polymers, etc.) to remove portions of the tubing in a desired pattern, the remaining portions of the metallic tubing forming the stent. Since the diameter of the stent is very small, the tubing from which it is made must likewise have a small diameter. Typically, the stent has an outer diameter of approximately 0.045 inch in its unexpanded configuration and can be expanded to an outer diameter approximately 0.1 inch or more. The wall thickness of the stent may be approximately 0.003 inch.
One method of cutting the tubing to produce a desired pattern is shown and described in U.S. Pat. No. 5,780,807 issued Jul. 14, 1998, and entitled “Method and Apparatus for Direct Laser-Cutting of Metal Stents”, the teaching of which are hereby incorporated by reference. This patent describes a method of producing a laser-cut stent wherein the tubing is cut into a desired pattern by means of a machine-controlled laser. The tubing is fixed in a rotatable collet fixture of a machine-controlled laser apparatus so as to position the tubing relative to the laser beam. The tubing is then rotated and moved longitudinally relative to the laser in accordance with a predefined set of machine-encoded instructions. In this manner, the laser selectively removes material from the tubing by ablation, thus cutting a desired pattern into the tube and forming the stent.
It should be appreciated that direct laser-cutting produces edges which are essentially perpendicular to the axis of the laser beam. Thus, the laser-cutting process produces stent cross-sections that are substantially square or rectangular. It should further be appreciated that to facilitate insertion of the stent through a patient's vasculature while at the same time minimizing risk to the patient, the expandable stents are preferably comprised of fine geometries and smooth edges. Thus, the edges of the rectangular cross-sections in the stent elements produced by direct laser-cuttings should-be rounded and/or smoothed.
One known technique for converting the generally square or rectangular cross-section of the stent elements into a more rounded, somewhat circular cross-section is electropolishing. For example, as described in above-referred-to patent, the stents may be electrochemically polished in an acidic aqueous solution comprised of, for example, sulfuric acid, carboxylic acids, phosphates, corrosion inhibitors, and a biodegradable surface-active agent. The bath temperature may be maintained at temperature of approximately 110° Fahrenheit to 135° Fahrenheit at an appropriate current density. Unfortunately, such electropolishing processes present certain problems. For example, such techniques may be cumbersome and messy. Furthermore, such processes are difficult to control resulting in, for example, stent-to-stent and in-stent variations in cross-section.
It should therefore be appreciated that it would be desirable to provide an accurate, reliable, and cost effective method for manufacturing low-profile, edgeless-geometry stents.